Field of the Invention
This invention relates to lightweight molds having low thermal inertia for bending glass sheets in pairs. The pairs are used to make bent laminated windshields. The present invention is especially suitable for mass production of automobile windshields of safety glass conforming to the American Standard Safety Code for Safety Glazing Materials for Glazing Motor Vehicles Operating on Land Highways Z 26.1 (1950), approved May 16, 1950 by the American Standards Association.
Automobile windshields of safety glass consist essentially of two matched sheets of curved glass such as commercial float glass of a soda-lime-silica composition, and an interlayer of a sheet of thermoplastic resin such as plasticized polyvinyl butyral sandwiched between the two glass sheets. The plastic interlayer is resilient and also adheres to the glass, especially when the glass sheets have matching shapes. Therefore, when a laminated safety glass windshield is broken on impact, the glass sheet fragments that form with jagged edges do not fly about. Instead, they remain attached to the plastic interlayer while the latter yields in response to impact against the windshield.
In present commercial practice, pairs of bent glass sheets used as components in laminated safety glass windshields are mass produced by the following series of steps:
(1) Cutting the pair of glass sheets while flat to their ultimate outlines differing slightly in size from one another;
(2) Arranging the sheets in pairs;
(3) Applying a parting material to the upper surface of the slightly larger sheet of each pair;
(4) Aligning each pair of sheets in face to face relation so that the slightly smaller sheet is above the other sheet of the pair and the parting material is between the sheets of the pair;
(5) Loading a pair of aligned sheets at a mold loading station on an outline bending mold having a shaping rail provided with an upwardly facing edge surface of concave elevation that conforms in elevation and plan outline to a shape slightly inward of the aligned margins of the pair of sheets after bending, said rail being connected to a relatively rigid reinforcing frame;
(6) Passing a succession of glass laden molds through a bending and annealing lehr where the glass sheets are heated to their deformation temperature so that they sag by gravity until the lower sheet conforms to the outline bending mold and the upper sheet of the pair sags to conform to the shape of the lower sheet;
(7) Cooling the glass sheets in a controlled manner from their deformation temperature through their annealing range to anneal the glass sheets as soon as the glass sheets attain their desired curvature;
(8) Further cooling the bent annealed sheets to a temperature at which the glass sheets can be handled; and
(9) Removing each pair of bent glass sheets from each mold in succession and returning the molds to the loading station for another bending and annealing cycle.
A vital factor in determining the rate of windshield production is the speed of the glass sheet pairs through the bending and annealing lehr. The lehr length determines the number of glass laden molds that can be handled simultaneously. The intensity of heat supplied per unit length of lehr and the amount of lehr heat absorbed by the molds and the lehr determine how much lehr heat is available to heat the glass sheets to their bending temperature and how rapidly the glass laden molds can be conveyed through the bending zone and arrive at the beginning of the cooling zone properly bent within the tolerances required by the customer. The lehr length also determines the time needed to return an unloaded mold from the exit of the lehr to the loading station at the entrance of the lehr.
For any given lehr to produce any given production pattern, the heating elements are controlled to impart a unique series of successive heating patterns designed to correlate with the rate of speed of the bending molds through the bending lehr to cause the glass sheets to conform exactly to the shaping surface of the mold at the time the heated glass enters the cooling and annealing zone. The series of heating patterns form a longitudinal and transverse temperature profile that is unique for each windshield production pattern. A record of the series of heating patterns of longitudinal and transverse temperature profile is kept for that lehr for each production windshield pattern for use whenever production schedules require additional production of a windshield pattern produced previously.
In recent years, automobile sales throughout the world have increased. The growth of automotive sales has required accompanying growth in windshield production. In the past, this increased production was accomplished by building new bending lehrs and running more molds per unit time through existing lehrs than previously by conveying glass laden molds more rapidly through the lehrs than previously while increasing the rate of heat applied to the glass to compensate for the shorter time of exposure of the glass to the hot environment of the bending lehr. In any given lehr, there is a maximum production rate dependent upon the maximum heat input that can be supplied to the bending lehr. Also, the production rate may be limited by the difference in thermal inertia between the shaping rail and the glass.
Thermal inertia as recited in this specification refers to the reluctance of a body to change its temperature in response to a change in environmental temperature. A massive metal shaping rail of large cross section has a higher thermal inertia than a shaping rail having a smaller cross section. Glass has a lower thermal inertia than stainless steel, the material used for shaping rails. Thicker glass sheets have more thermal inertia than thinner glass sheets. Furthermore, the difference in thermal inertia between glass sheets of a given thickness and a stainless steel shaping rail can be minimized by reducing the cross section of the shaping rail. The need for shaping rails of less thermal inertia has been intensified with the reduction of glass sheet thickness in windshields from a nominal 1/8 inch (3.2 millimeters) to 0.090 inch (2.3 millimeters). The thinner glass sheets of present day windshields have less thermal inertia than those included in earlier commercial windshields.
The difference in thermal inertia of the shaping rail and the glass sheet portion in contact with the shaping rail during the heating step needed to bend glass sheets is associated with the phenomenon of chill cracking. Chill cracking occurs when a relatively cool shaping rail portion contacts a relatively hot glass sheet portion during the heating of the glass sheet to its bending temperature. The portions of the relatively low thermal inertia glass in contact with the relatively high thermal inertia shaping rail develop tension stresses when the glass reaches a higher temperature than the shaping rail before the glass sheet reaches the annealing temperature range. Since glass is notoriously weak in tension, surface fissures or cracks are likely to be formed under such circumstances. When the mold is preheated to a temperature sufficient to compensate for its higher thermal inertia than that of the glass so as to avoid having the glass sheet contact the shaping rail at a temperature sufficiently hotter than that of the shaping rail portions in engagement with the glass sheet, tension stresses that tend to cause failure of the glass are usually avoided. However, it is inefficient to have to heat a bending mold prior to each bending cycle in order to reduce the tension stresses in the glass that cause the glass sheets to break. In addition, the prior art found it difficult to reduce the thermal inertia of the shaping rail without causing the mold to lose its structural rigidity.
In the past, the mass of the bending molds used to support the glass sheet for conveyance through the bending lehr was such as to limit the rate of throughput of glass sheets bent in a mass production operation. The relatively heavy weight of the metal bending molds compared to the weight of the glass sheets supported thereon for bending resulted in a relatively inefficient use of the thermal energy imparted into the bending lehr. Previous attempts to reduce the mass of the molds used to shape glass sheets resulted in disappointment because lightening the mass of the molds also reduced the rigidity of the mold so that the molds tended to distort. It would be beneficial to the glass sheet bending art to develop molds of lighter weight than previously that did not distort due to exposure to variations in temperature as a consequence of the glass sheet bending operation.
It also would be beneficial to the glass sheet bending art to develop a method of making outline molds conforming in elevation and outline to the shape desired for the bent glass sheets involving fabrication techniques that do not require localized intense heating such as is necessary for a welding operation which has been used to connect the relatively lightweight shaping rails of the bending molds to more massive mold reinforcing means that require a relatively rigid frame.